1. Field of the Invention
Novel injection-moldable articles of a hydrogenated triblock copolymer such as nipple bodies (for use on a bottle to feed an infant), teething rings, or pacifiers; and extrudable medical tubing. Such articles of the highest quality are currently molded from a relatively soft silicone (polysiloxane) which is substantially permeable to visible light. The articles are required to have properties which allow the articles to be mouthable or bitable by an infant. Such a silicone is not recyclable. An injection-moldable thermoplastic elastomer (“TPE”) derived from a styrene-conjugated diene-styrene triblock blended with a polyolefin and tailored to mimic the physical properties of the bite-resistant silicone articles, is recyclable.
2. The Problem
The aforementioned articles are currently molded from one, or a blend of polysiloxanes (silicones), the blend chosen to provide desirable targeted properties of tear strength, tensile strength, softness and lack of haze (or clarity), as well as to be able to withstand boiling water for an hour without any measurable distortion. But a silicone is not recyclable into fresh product because it is not thermoplastic; and, inability to injection mold a silicone adds to cost of production of formed articles. The goal is to substitute a thermoplastic, injection-moldable copolymer for the silicone while providing essentially the same physical properties as those of the targeted silicone.
3. Addressing the Problem
Moldable articles such as nipple bodies, teething rings and pacifiers are typically made from a silicone which has specified desired properties; but molding a silicone article is relatively expensive since it is not thermoplastic. The desired properties of a typical “nipple grade” silicone are as follows: tensile strength near 6.9 MPa (1000 psi); tear resistance near 35 N/mm (200 lb/in); heat resistance to withstand sterilization temperatures near 100°C. without losing dimensional stability; hardness no greater than Shore A 65; and lack of substantial haze to provide “see-through” clarity so as to provide visual indication of cleanliness. Though haze and clarity are measured differently, only haze is measured herein, on the basis that substantial lack of haze, less than 20%, provides a substantially transparent injection molded material. The combination of tear strength and tensile strength provides bite-resistance.
A silicone having the aforesaid desired properties is used in premium mouthable articles because a silicone can be formulated with the desired physical properties and also be substantially transparent. Being able to see through a nipple body makes it easy for a caregiver to determine that the nipple body appears to be clean, and, when in use to feed an infant, to determine whether the infant is drawing liquid through the nipple or teat. By “substantially transparent” is meant that the molded composition has substantially no haze, that is, less than 20%, typically from 15% –20%, for a plaque 1.7 mm thick, measured with a BYK Gardner Micro Tri-gloss 4525 meter and ASTM D1003 test procedure. Haze below 20% permits a “see-through” property sufficient to allow one to read black letters printed in 12 point font on a white surface through a thickness of about 1.7 mm (0.125 in) held 15 cm away from the surface. Though rubber latex nipple bodies (hereafter “nipples” in deference to common usage) are less expensive, they typically have a haze>20% and are deemed substantially opaque. Haze below 20%, i.e. <20%, is deemed equivalent to acceptable clarity.
TPE block copolymers having blocks of a vinylaromatic monomer and a conjugated diene monomer (referred to herein as a “SBC” irrespective of the monomers chosen, because typically styrene is the vinylaromatic monomer chosen) and high melt viscosity, are routinely blended with a polyolefin such as polypropylene (PP) to provide an injection-moldable blend but it is difficult to predict the properties of the blend. For example, a blend of styrene/butadiene/styrene or a styrene/isoprene/-styrene block copolymer with PP, by mechanical kneading and extruding, results in so many variables coming into play that the distribution of rubbery domains is not predictable. Not least of these variables is the ratio of SBC to PP; when there is more PP by weight than SBC, the properties are different from those when there is more SBC than PP. Moreover, the physical properties of the SBC are mainly determined by its molecular weight (or melt index), the ratio of styrene to rubber blocks, and the extent (mole %) of vinyl linkages in the rubbery midblock. It is known that the higher the molecular weight of a SBC relative to another, all other criteria being the same, the higher the tensile strength and tear strength of a blend with PP, used in same amount and having the same melt index. But one cannot predict other properties of the blends, nor make a close estimate of the improvement in tensile and tear strengths. Thus to get high tensile and tear strengths one is inclined to choose a SBC having a number average molecular weight in excess of about 150,000(measured by gel permeation chromatography (GPC), (hereafter “Mn” for brevity). But such “high Mn” substantially transparent commercially available SBCs are not substantially transparent when blended with sufficient PP to form a continuous phase so as to have a hardness below Shore A 65. It is also known that SBCs having a relatively low Mn, less than about 150,000are substantially transparent, but they have unacceptable tensile and tear strengths when blended with sufficient PP to form a continuous phase so as to have a hardness below Shore A 65.
The foregoing is also true for a substantially fully hydrogenated SBC (referred to herein as a “HSBC”) blended with polypropylene, except that the clarity of a “high vinyl” HSBC having a Mn in the range from about 100,000 to 170,000 is better than that of an unhydrogenated SBC having the same Mn.
By “high vinyl” is meant that at least 51 mole % (percent) of the butadiene midblock is polymerized at the 1,2-position, and at least 51 mole % of the isoprene, if present, is polymerized at the 3,4-position by “driving” the polymerization with addition of a polar compound, as is well known in the art; typically the maximum in each case is 90 mole %. Such HSBCs are referred to as “high vinyl” HSBCs whether either butadiene or isoprene, or both, are present in the midblock.
A single high vinyl HSBC having Mn in the range above about 170,000, when blended with a minor proportion by weight of a homopolymer of a lower olefin, or a copolymer of lower olefins having from 2 to 5 carbon atoms, to provide desirable heat resistance and processability, lacks both essential softness and clarity (haze<20%). However, when a substantially transparent high vinyl HSBC with Mn lower than about 170,000, preferably in the range from about 60,000 but less than 130,000, is blended with the same polyolefin, typically polypropylene, it remains substantially transparent, but is still too hard having a Shore A hardness greater than 65 (>65).
In each blend of high vinyl HSBCs in the Mn range from 60,000 to 170,000, the minor proportion by weight of PP in the blend is sufficient to form a continuous phase which accounts in large part for the good physical properties of the blend, including desirable heat resistance. However, as stated, the good physical properties are typically accompanied by unacceptable hardness, i.e. Shore A hardness>65, thus suggesting the blend be plasticized. But many blends, if plasticized or softened enough to fall in the required range of from Shore A 45 to less than Shore A 65 when injection molded or extruded, the plasticizer (or softening agent) tends to migrate to the surface and adversely affects both the feel and the usability of the molded product and/or the blends have relatively poor tensile and tear strengths.
U.S. Pat. No. 5,544,766 discloses a nipple for a baby bottle which is fabricated from a flexible material, “usually latex, vinyl, silicone or thermoplastic elastomers (TPEs)”. There is no indication in the '766 disclosure as to what particular TPE or blend of TPEs might be used as an acceptable substitute for a substantially transparent silicone for which no physcial properties are provided. Since there is no suggestion as to how to arrive at a suitable TPE substitute for the unspecified silicone, much less specifications to enable one skilled in the art to produce a satisfactory TPE substitute, it is evident that it would require an undue amount of experimentation to arrive at those specifications. The invention disclosed herein is the result of having done the necessary experimental work.
A commonly available SBC is said to be partially hydrogenated when it has from about 20 to 50% olefinic unsaturation; but when the SBC is further hydrogenated to a HSBC containing less than 10% olefinic unsaturation, it is commonly regarded in the art as being substantially fully hydrogenated. The physical properties of an unhydrogenated SBC having a molecular weight essentially identical to that of a HSBC (substantially fully hydrogenated SBC) are typically quite different from those of the HSBC due to the interaction of the rubbery phases which have different chemical structures. In this invention, unhydrogenated SBCs are found to have unacceptable properties. In particular, only substantially fully hydrogenated (A)-(B)-(A) type “HSBC” wherein (A) represents a polystyrene block and (B) represents a polybutadiene or polyisoprene block, or a “mixed” poly(butadiene-isoprene) block, having less than 30% unsaturation, preferably less than 20% unsaturation, lend themselves to being incorporated in a heat stable, oxidation-resistant TPE of this invention.
It is known that the morphology of hydrogenated (A)-(B)-(A) type HSBC changes as the ratio of (A) to (B) changes; and it changes further depending upon the amount of polypropylene blended with the HSBC; but there is no teaching as to how such morphology might affect tear strength, tensile strength and/or clarity when various mixtures of HSBCs blended with polypropylene are molded. Thus, knowing the tensile and tear strengths of a specific hydrogenated (A)-(B)-(A) block copolymer having a specified molecular weight blended with a chosen amount of polypropylene, and knowing the specified ratio of molecular weights of the (A) and (B) blocks, it is not possible to estimate, with any reasonable degree of certainty, the tensile strength, tear strength and clarity of another block copolymer with a different molecular weight but the same (A)/(B) ratio of molecular weights and the same amount of polypropylene; or, with a different (A)/(B) ratio and the same molecular weight and amount of poly-propylene; making an estimate is even more difficult when that block copolymer blended with polypropylene is molded with relatively large amounts of additional ingredients.
The degree of uncertainty is further increased when two block copolymers with different characteristics are blended, and two of the added ingredients (polypropylene and plasticizer) are added in relatively large amounts to affect phase distribution and domain size. The challenge of providing an appropriate blend for a particular article to be molded, has been addressed in the prior art.
To provide a moldable cover for a cavity containing a deployable air bag, U.S. Pat. No. 6,106,011 teaches a mixture having a specified ratio of two hydrogenated block copolymers, each having a molecular weight in a specified range and a specified proportion of blocks, which mixture is further modified by a polyolefin having a specified melt flow rate, and optionally with a hydrocarbon oil. In the unlikely event that one seeking to mold a nipple for a baby's bottle would stumble upon a cover for an air bag and divine that it may have a suggestion to help address the problem of mimicking a silicone nipple with a mixture of HSBCs, he will not find one.
U.S. Pat. No. 6,399,696 teaches a recipe useful to make a gasket material, comprising 100 parts by weight of a HSBC of a vinyl aromatic compound and a conjugated diene compound mixed with from 50 to 1000 parts of a non-aromatic base softening agent having a kinematic viscosity at 40° C. of at least 100 mm2/sec, and from 1 to 100 parts of polypropylene or a propylene copolymer. The end product is a TPE which has a Shore A hardness greater than 65, and therefore unacceptable for the articles of this invention. Understandably, there is no indication in the '696 patent as to what the tensile and tear strengths are of the compositions they teach since neither property is of concern in a gasket material.
No prior art reference teaches how to choose the particular ingredients for a TPE which when injection molded, will yield an article with targeted physical properties, particularly a combination of dimensional stability at 100° C., hardness, clarity (lack of substantial haze), tensile and tear strengths. Though blends of HSBCs are modified with a polyolefin and plasticized in the prior art, it does not teach how to choose appropriate HSBCs having defined properties, or how to combine these HSBCs with what other ingredients, in a recipe for a blend which will yield an injection molded article with specific, targeted properties. In particular there is nothing to suggest that a blend of a high vinyl HSBC, when blended with polypropylene and a plasticizer would provide the foundation for an injection-moldable block copolymer which could mimic the properties of a silicone.
Hydrogenated high 1,2-HSBCs are disclosed in U.S. Pat. No. 5,777,031 to Djiauw et al the disclosure of which is incorporated by reference thereto as if fully set forth herein. The high 1,2-polymers, which had end blocks each having a Mn of at least 20,000 and Mn>130,000 for the entire polymer, were found to have better softness than polymers with the same ratio of styrene to butadiene but a 1,2-addition lower than 51 mole percent. But blends of such high 1,2-HSBCs with PP and mineral oil, failed to have adequate clarity, that is, minimal haze (<20%), if they were in the required range of hardness, or had the required strength. Nor is there any suggestion in the '031 patent that a “high vinyl” HSBC blend might be substantially transparent provided that its resultant effective Mn was below 130,000.
By “resultant effective Mn” is meant the Mn arrived at by combining the Mn of each HSBC in the proportion it is present in the blend, divided by the number of HSBCs in the blend.
Testing indicated that no single high vinyl HSBC having Mn>130,000, when blended with PP and plasticized, without blending with a dynamic vulcanizate of an engineering thermoplastic and an acrylate rubber, provides properties mimicking those of a soft, tough, transparent silicone such as is used in a nipple for feeding an infant.